Television receiver



E?, QMTZ.

TELEVI S ION RECE l VEP Filed March 50, 1,940

CHESS/15 Snvenfors Cttorneg Patented Mar. 3, 1942 TELEVISION RECEIVER Kenneth A. Chittick, Haddonfield, and Randall C. Ballard, Audubon, N. J., assgnors to Radio Corporation of America, a corporation of Delaware Application March 30, 1940, Serial No. 326,852

(Cl. 17E-7.3)

3 Claims.

Our invention relates to television receivers or the like, and particularly to the high voltage supply unit for the cathode ray tube in such receivers.

A high voltage unit of this character includes a step-up transformer for transforming the line voltage, such as 110 volts, to a voltage of several thousand volts. This transformer must be safe from voltage breakdown at several times the normal operating voltage, the present underwriters requirement being that the transformer be able to operate with three times normal voltage across the primary without any voltage breakdown between the primary and secondary windings. If a transformer is designed with enough insulation to withstand the above-mentioned test, it substantially increases the size and the cost of the transformer.

It is, accordingly, an object of our invention to provide an improved means for and method of preventing voltage breakdown of a transformer in a circuit of the above-described character.

In a preferred embodiment of our invention, the transformer is of the type in which one end of the secondary winding is adjacent to the primary winding while the other end of the secondary winding is spaced a substantial distance therefrom. In accordance with our invention, a condenser of large capacity as compared with transformer distributed capacity is connected between said one end of the secondary winding and the primary winding whereby, as will be explained hereinafter, substantially the entire voltage difference between the primary and secondary windings appears at the other end of the secondary. Thus, the high voltage difference appears at a point where there is no danger of voltage breakdown.

The invention will be better understood from the following description, taken in connection with the accompanying drawing, in which- Figure 1 is a circuit diagram of a portion of a television receiver embodying our invention,

Figure 2 is a View, partly in cross section, of the high voltage transformer shown in Figure 1, and

Figure 3 is a circuit diagram of a modification of our invention.

Referring to Fig. 1, there is shown a portion of a television receiver comprising a cathode ray tube I, a high Voltage supply unit 2 and the receiver chassis 3. The tube I is of the usual electrostatically focused type requiring high voltages on its rst and second anodes 4 and B, respectively.

The voltage supply unit 2 comprises a high -f voltage step-up transformer I having a primary winding 8 and a secondary winding 9, a rectifier I I having a filament heated from a transformer winding I2, a smoothing lter consisting of a series resistor I3 and shunt condensers I4 and IB, and a voltage divider II. The cathode I8 and the anodes 4 and I3 of the cathode ray tube are connected to suitable points on the voltage divider I'I.

In practice, it is found that many receivers are operated with the chassis 3 ungrounded. Since the chassis is commonly used as a bus connection in wiring up the receiver, as in the example illustrated where the cathode IB and the low potential side of the voltage supply are connected to the chassis, the chassis may acquire a high voltage above ground if there is a Voltage breakdown of the transformer I. The importance of avoiding a transformer breakdown under these circumstances will be appreciated.

The construction of the transformer 'I is shown in Fig. 2 where the primary coil 8 is wound on one leg of a closed iron core 2I. The secondary winding 9 is wound over the primary coil 8 with the secondary winding starting at the point C adjacent to the primary winding and ending at the point B spaced a maximum distance from the primary coil. It will be apparent that it will require a much higher voltage to cause breakdown between the point B and the primary than between the point C and the primary.

In accordance with our invention, a condenser C3 (Fig. 1) of large capacity as compared with the transformer distributed capacity is connected between the primary winding 8 and the end C of the secondary winding 9. The effect of this is to bring the secondary end C to a potential close to that of the primary Winding 8 while the potential of the secondary end B is nearly doubled. However, this end B is spaced so far from the primary winding that there is no danger of voltage breakdown.

The reason that the condenser C3 has the above-described effect will be understood from a consideration of the distributed capacity between the transformer coils 8 and 9. For the purpose of illustration, assume that the distributed capacity between these coils is the same at both ends of the secondary, as indicated by the equal capacities CI and C2 in Fig. 1. Then, in the absence of condenser C3, a phantom ground exists at the center point A of the secondary winding 9.

Assuming a case where 6000 volts is to be obtained from a 11G-volt line for the second anode of the cathode ray tube, the application of three times normal voltage to the primary during a transformer test would bake the secondary end C 9000 volts below ground and the end B 9000 volts above ground at a given instant. Since the primary winding is either at, or substantially at, ground potential, this means that without the condenser C3 there is a voltage diierence of 9000 volts between the secondary end C and the primary winding.

By employing the condenser C3 as described, the voltage distribution on the secondary is changed to bring the high voltage dierence to a point in the transformer where the maximum available voltage cannot cause voltage breakdown.

It may be noted, by way of example, that in one embodiment of the invention the condenser C3 was a 0.004 microfarad, 100G-volt mica toothpick condenser. critical.

In Fig. 3, there is shown a variation of the invention which prevents the chassis from having more than one-half line voltage on it. In Figs. l and 3 like parts are indicated by the same reference numerals.

It will b-e noted that, in the circuit of Fig, l, if the input leads of the transformer 1 are plugged into the 110-Volt line with the plug turned so that the condenser C3 is connected to the ungrounded side of the line, instead of to the grounded side as indi-cated in the drawing, the chassis 3 will have 110 volts applied to it through the condenser C3. While there may be no serious objection to this, it is preferable to have the chassis at the lower voltage which is made possible by the arrangement shown in Fig. 3.

In Fig. 3, the normal A.C. voltage of the chassis 3 with respect to ground is one-half the line voltage. This result is obtained by connecting the end C of the secondary 9 to both sides of the 110-volt line through the condensers 26 and 21, each of which has large capacity as compared with the distributed capacity of the transformer 1. In this way, voltage breakdown of the transformer 1 is prevented in the same manner as described in connection with Fig. 1 and, at the same time, the chassis is at a lower A.C. voltage with respect to ground.

We claim as our invention:

1. In a television receiver having a chassis which may be ungrounded, a high voltage unit having its low Voltage side connected to said chassis and comprising a high voltage trans- Its capacity value obviously is not former having a primary winding and a secondary winding, the turns in said secondary winding starting adjacent to said primary winding and ending at a point comparatively remote from said primary winding whereby the first turns only are adjacent to said primary winding, and a condenser connected between said primary winding and a point on said secondary winding near the start thereof, the capacity of said condenser being large compared with the distributed capacity between said primary and secondary windings whereby most of the voltage difference between said'windings is at the secondary turns remote from the primary winding.

2. In a television receiver having a chassis which may be ungrounded, a high voltage unit having its low voltage side connected to said chassis and comprising a high voltage transformer having a primary winding and a secondary winding, one end of said secondary Winding being closer to said primary lwinding than the other end whereby the turns of said other end are comparatively remote from said primary winding, and a condenser connected between said primary winding and said one end of said secondary winding, the capacity of said condenser being large compared with the distributed capacity between said primary and secondary winding'whereby most of the voltage difference between said windings is at said remote turns.

3. In a television receiver having a chassis which may be ungrounded, a high voltage unit having its low voltage side connected to said chassis and comprising a high voltage transformer having a primary winding and a secondary winding, the turns in said secondary winding starting adjacent to said primary winding and ending at a point comparatively remote from said primary winding whereby the rst turns only are adjacent to said primary winding, a condenser connected between one end of said primary winding and a point on said secondary winding near the start thereof, and a second condenser connected between the other end of said primary winding and said last-named point on the secondary winding, the capacities of each of said condensers being large compared with the distributed capacity between said primary and secondary windings, whereby most of the voltage difference between said windings is at the secondary turns remote from the primary winding.

KENNETH A. CHITTICK. RANDALL C. BALLARD. 

